This invention relates to a method of controlling an exhaust gas recirculation valve (hereinafter referred to as an EGR valve) disposed in an exhaust gas recirculation system.
FIG. 1 is a conventional schematic diagram showing an arrangement of a control valve 11 as an EGR valve disposed in an exhaust gas recirculation passage c which communicates an exhaust passage xe2x80x9caxe2x80x9d of an engine E with an intake passage b.
The method of controlling the EGR valve involves driving and controlling a stepping motor M of a hybrid PM type 4-phase construction or the like by an engine control unit (hereinafter referred to as ECU) 51, and controlling the opening and closing of the control valve 11 by this stepping motor M. An open-loop control of the stepping motor M by a stepping angle contributes to control over the degree of the opening of the control valve 11.
Such a control method using this kind of stepping motor M imposes restrictions on the control over the degree of the opening of the control valve 11 because the degree of the opening of the control valve 11 can be controlled only by the stepping angle of the stepping motor M. The control valve 11 has a limited resolution for the controllable opening. In addition, the stepping motor M has a limited open-loop control response characteristic due to the possible occurrence of a stepping-out phenomenon. Once the stepping-out has occurred, the reliability falls as an error is still contained uncompensated in the control amount.
To this end, the conventional control method of an EGR valve involves giving a predetermined return torque to the control valve 11 in the control valve closing direction and, by a unidirectional driving of a direct current (DC) motor M (hereinafter referred to as a motor M), giving a motor torque to vary the control valve 11 in the opening direction, and opening and closing the control valve 11 by the balance of these torque.
An arrangement is described in Japanese Published Patent Feeding No. 159405/1999. This arrangement includes an open loop control system for controlling an open loop of the motor M such that a motor torque is generated in correspondence with the target opening and closing position of the above control valve 11; and a feedback control system for feedback-controlling the motor M based on a deviation between input data corresponding to the target opening and closing position of the above control valve 11 and detected data of the current opening and closing position of the control valve 11.
FIG. 2 is a characteristic diagram showing the relationship between a motor torque and an opening and closing position of a control valve in an EGR valve of torque balance drive system.
First, the driving system using this motor M will be described. In case the degree of the opening of the control valve 11 is feedback-controlled by the motor M, the generated torque of the motor M is continuously controlled by feeding back the degree of the opening of the control valve 11 through unintermitted detection with a position sensor such as a sliding resistor type. Theoretically, the continuous control over the generated torque of the motor M promotes infinite reduction of the resolution of the controllable opening of the control valve 11.
This kind of method of controlling the EGR valve using the motor M adopted a so-called torque balance method. The method involves giving a predetermined return torque in the closing direction by means of a spring as urging means, giving a motor torque variable in the opening direction by driving the motor M in the opening direction, and determining the valve opening position by the balance of these torque.
In case this kind of control method is adopted, since the EGR valve is constantly given the return torque, the opening and closing positions (shift amount) vary in correspondence with the inclination of lines A, B having a hysteresis characteristics due to friction as shown in FIG. 2.
Here, line A indicates an operating characteristic when the control valve 11 is opened by increasing the motor torque, and line B an operating characteristic when the control valve 11 is closed by reducing the motor torque. The inclination of the operating characteristics A, B varies depending on the spring constant of the spring to give the return torque, and the operating characteristics A, B shift to the right or left in FIG. 2 depending on the magnitude of the set torque.
Now, in order to control the control valve 11 having this kind of operating characteristics, suppose that a method is admitted, in which the motor is under the control of a P(proportional) I(integral) control based on a deviation between the input data corresponding to the target opening and closing position of the control valve 11 and the detected data of the current opening and closing position of the control valve. In this case, owing to the relation of the operating characteristics A, B as shown in FIG. 2, it becomes difficult to stabilize the control valve 11 at the target opening position.
In other words, in order to open the control valve 11 to the target opening position by increasing the motor torque, the P gain and the I gain must be increased to take control along the operating characteristic A shown in FIG. 2. However, when the motor torque is increased by the PI control under the control of this kind, the deviation of the opening position of the control valve becomes xe2x80x9c0xe2x80x9d as soon as the control valve 11 is opened to the target opening position. The P component thus becomes xe2x80x9c0xe2x80x9d and the I component is cleared, with the result that the control valve 11 begins to close by the return torque.
FIG. 3 is a characteristic diagram showing the relationship between the time and the operating position of a motor shaft.
At an initial stage in which the control valve 11 begins to close (at the time the deviation is small), the P and I components are both small and therefore the motor torque cannot overwhelm the return torque, with the result that the deviation becomes large. Thereafter, even if the deviation becomes large to a certain degree, the motor torque and the return torque balance with each other, the closing operation of the control valve 11 cannot stop abruptly due to the inertia of the motor M. The control valve 11 thus cannot be opened immediately. If the gain is made large such that a relatively large motor torque is generated even at the time the deviation is small, there will be a vicious cycle that incurs an increase of the overshooting and undershooting as shown in FIG. 3.
FIG. 4 is a longitudinal sectional view of the EGR valve.
The arrangement implementing a method of controlling the control valve 11 in a so-called torque balance drive system using the motor M will now be described, with due consideration of the above circumstance.
Referring to FIG. 4, reference numeral 1 denotes a valve body having therein a passage forming a part of an exhaust gas recirculation passage c which is interposed in a recirculation system of the exhaust gas. By upwardly moving the control valve 11 to contact it with a valve seat 12 as shown in FIGS. 4 to 6 the exhaust gas recirculation passage c is closed. Conversely, by downwardly moving the control valve 11 to depart it from the valve seat 12 the exhaust gas recirculation passage c is opened.
Reference numeral 2 denotes a motor case for housing therein the motor M. Inside this motor case 2, reference numeral 21 denotes a rotor around which a coil 22 is wound, and reference numeral 23 a yoke with a magnet 24. The lower end of the rotor 21 is rotatably supported on the valve body 1 by a bearing 27.
Inside the rotor 21, a motor shaft 31 is screwed. The motor shaft 31 is prevented from rotating by a guide bush 13 in the body 1. It therefore follows that the motor shaft 31 moves in the upward and downward direction depending on the amount of rotation of the rotor 21. A valve shaft 14 is held in contact with the lower end of the motor shaft 31, and an intermediate portion of which is guided by a guide seal 15 and a guide plate 16 so as to be movable in the upward and downward direction relative to the valve body 1. The control valve 11 is attached to the lower end of the valve shaft 14.
Reference numeral 17 denotes a guide seal cover. Between a spring sheet 18 mounted on the upper end of the valve shaft 14 and the guide plate 16, a return spring 19 is interposed for urging the valve shaft 14 in the upward direction, i.e., in the closing direction to urge the control valve 11.
The control valve 11 thus constituted is driven by the torque balance system as described above. In other words, the control valve 11 is given a predetermined return torque by the return spring 19 as urging means in the control valve closing direction and is also given a variable motor torque in the control valve opening direction by driving the motor M. By the balance of these torque, the control valve 11 is opened and closed.
FIG. 5 is a block diagram of an ECU apparatus for carrying out the method of controlling the EGR valve in the so-called torque balance driving system using a motor.
Referring to FIG. 5 reference numeral 50 denotes a microcomputer as a control part for determining the motor driving voltage, and reference numeral 52 a battery. Reference numeral 53 denotes a motor driving voltage converting part for converting the output of the control part 50 to supply the converted output to the motor M, which includes a Zenor diode 53a; a diode 53b for passing a unidirectional current only to the motor M; a field-effect transistor (FET) 53c; and an interface 53d disposed between the control part 50 and the FET 53c. Reference numeral 56 denotes a regulator for generating a driving voltage (5V) of the control part 50.
The control part 50 receives as inputs through interfaces 58, 59, respectively, a detected signal from an operating property sensor 57 mounted on each part of the vehicle such as a crank angle sensor or the like, as well as a detected signal from the position sensor 40. The position sensor 40 in this example is provided with a movable contact part 42 movable on a resistor 41 to which a constant voltage (5V) is applied from a voltage supply part 60. With the movement of the movable contact part 42 as a result of the rotation of the rotor 21, a voltage corresponding to the rotating position of the rotary shaft 31 is outputted, as a detected signal, from the movable contact part 42.
Further, the above motor driving voltage converting part 53 switches on and off the voltage to be applied to the motor Mat a constant cycle. By a pulse-width modulation (PWM) signal whose pulse width is determined depending on the ratio of the on-time and the off-time per a cycle (driving duty), the FET 53c is switched to control an average driving voltage to be applied to the motor M.
FIG. 6 is a block diagram showing a control part in the ECU.
Referring to FIG. 6, reference numeral 61 denotes a target position computing part for computing an optimum opening and closing position of the control valve 11 based on the detected signal of the operating property sensor 57, which outputs a voltage corresponding to the target position (hereinafter referred to as a xe2x80x9ctarget valuexe2x80x9d). Reference numeral 62 denotes an analog-digital (A/D) converter for performing an A/D conversion of a detected signal of the position sensor. 40, which outputs a voltage corresponding to the current position (hereinafter referred to as a xe2x80x9ccurrent valuexe2x80x9d). Reference numeral 71 denotes an adder-subtractor for adding or subtracting the target value and the current value. Reference numeral 63 denotes a PI controlled variable computing part for computing and outputting the PI controlled variable (voltage) by combining the proportional component (P component) and the integral component (I component) based on the deviation between the target value and the current value. Reference numeral 64 denotes a driving duty computing part for computing the duty to be supplied to the motor M based on the output of the PI controlled variable computing part 63.
Then, the operation of the control part will be described.
When a target value is given from outside, the current value detected by the position sensor 40 and the above target value are added or subtracted by the adding-subtracting part 71 to obtain a deviation. The PI controlled variable computing part 63 computes a PI controlled variable from the obtained deviation and outputs the computed PI controlled variable to the driving duty computing part 64. The driving duty computing part 64 computes the driving duty based on the PI controlled variable and outputs the computed driving variable to the motor M.
Such a conventional method of controlling the EGR valve, in case the valve shaft ceases to move due to biting, clogging of dirt, or the like, also ceases to move the motor shaft which is in abutment with the valve shaft. As a result, the motor extraordinarily generates heat caused by the applied electric current and is in a danger of a seizure. The conventional method of controlling the EGR valve cannot be quickly and appropriately deals with this kind of accident.
The invention has been made to solve the above and other problems and an object thereof is to provide a method of controlling an EGR valve which is able to prevent previously the occurrence of a seizure in the event the valve shaft has ceased to move.
A method of controlling an EGR valve according to the invention in which a valve-open position is controlled by a torque balance between a return torque given by urging means in the valve closing direction and a motor torque given by a motor in the valve opening direction, comprises the step of stopping power feeding to the motor if an open close valve failed to operate despite a full (100%) continuous power feeding to the motor for a predetermined period of time or more.
Therefore, even if a valve shaft ceases to move due to biting, clogging of dirt, or the like, the occurrence of a seizure of the motor can be prevented previously.
In the method of controlling an exhaust gas recirculation valve according to the invention further comprises the step of driving the motor in the valve closing direction and thereafter driving the motor in the valve opening direction again if the open-close valve failed to operate despite a full (100%) continuous power feeding to the motor for a predetermined period of time or more.
Therefore, even in case the valve shaft ceases to move due to biting, clogging of dirt, or the like, it is possible to remove the clogging of dirt, or the like to thereby drive the valve shaft again.
In the method of controlling an exhaust gas recirculation valve according to the invention further comprises the step of gradually decreasing the driving current to the motor from a valve-opening position exceeding a target position in a steady state so as to cause the open-close valve to move in the valve closing direction by the return torque until the valve-opening position closes to the target position.
Therefore, it is possible to open the valve to the target valve-open position quickly and stably with a small amount of driving electric current.
In the method of controlling an exhaust gas recirculation valve according to the invention further comprises the step of lowering a driving frequency for opening and closing a power feeding circuit to the motor if an ON-duty to open and close the power feeding circuit becomes larger than a set value.
Therefore, when the valve shaft becomes difficult to move and the driving power to the motor increases, the opening and closing frequency of the power feeding circuit for the motor can be decreased previously. As a result, it is possible to prevent the damages to the switching element due to heat generation as well as the seizure of the motor.
FIG. 1 is a conventional schematic diagram showing an arrangement of a control valve as an EGR valve disposed in an exhaust gas recirculation passage c which communicates an exhaust passage xe2x80x9caxe2x80x9d of an exhaust engine E with an intake passage b.
FIG. 2 is a characteristic diagram showing the relationship between a motor torque and an opening and closing. position of a control valve in an EGR valve of torque balance drive system.
FIG. 3 is a characteristic diagram showing the relationship between the time and the operating position of a motor shaft.
FIG. 4 is a longitudinal sectional view of the EGR valve.
FIG. 5 is a block diagram of an ECU apparatus for carrying out the method of controlling the EGR valve in the so-called torque balance driving system using a motor.
FIG. 6 is a block diagram showing a control part in the ECU.
FIG. 7 is a flow chart explaining the control method of the invention.
FIG. 8 is a subflow chart explaining a computation of the manipulated variable at step St1 in FIG. 7.
FIG. 9 is a subflow chart explaining an EGR control during fault condition at step ST8 in FIG. 7.
FIG. 10 is a subflow chart explaining an EGR transistor duty output of the EGR transistor duty at step ST7 in FIG. 7.